The present invention relates to electrical test devices and, more particularly, to a hand-held device designed to test circuits of the type having a two-part plastic housing and an insert which serves to structurally reinforce the walls of the housing and, in one embodiment, additionally provides strain protection for an extending lead, as well as a mount for a spare bulb.
Devices for testing electrical circuits have been long known in the art and have taken a variety of different forms. However, circuit test devices to which the present invention pertains are of the hand-held type which, in general, comprise a plastic housing designed as a handle to be grasped by the hand of the operator. A test probe or pin extends from the handle and is used to make contact with specific areas of the circuit under test. An indicating device, and associated components, are situated within the housing. A wire lead, usually having an alligator clip or the like on the end thereof, also extends from the housing. The lead is connected to one portion of the circuit under test and the hand-held probe is utilized to contact other portions of the circuit. If the circuit under test is functioning properly, the indicating device, such as a small incandescent bulb or the like, will be energized. The housing is fabricated of translucent or transparent plastic, such that the bulb is visible.
Test devices of this type are designed either to utilize the power of the circuit under test, or to provide the necessary power to energize the circuit during the test. In the latter case, the housing from which the probe extends is provided with a cavity into which a power source, such as a battery or the like, is received. The battery provides the electrical power to energize the circuit for the test.
Both types of circuit test devices employ a means for mounting an incandescent bulb or lamp within the housing and the self-powered type additionally includes a means for mounting a battery within a cavity in the housing. It is, therefore, necessary to construct the housing such that access to these components is possible to permit replacement thereof. For this reason, conventional circuit test devices of this type normally consist of a plastic housing, large enough to carry the necessary components, which is generally cylindrical in form, so as to provide a handle-type member which can be easily grasped and manipulated by the user. The housing is formed in two parts which are removably connectable so as to permit the housing to be easily opened for access to the components situated therein.
A common manner of joining the housing parts is to form overlapping sections and to externally thread the section of one part and internally thread the section of the other part, such that the overlapping sections can be screwed together. The screw threads are formed in each part by providing a helical groove in the cylindrical wall thereof. In the section of the first part, the groove is formed in the exterior of the wall, such that the surface of the thread is aligned with the surface of the wall of the non-threaded section. Thus, the structure of the wall in the threaded section is weakened because certain portions thereof are thinner, by a distance equal to the depth of the groove, than the remainder of the wall.
The parts are joined together at the middle of the housing. When the device is used, the forces concentrate at this point. If the externally threaded section is not sufficiently strong to withstand these forces, same will collapse, destroying the usefulness of the device.
It is, of course, possible to increase the thickness of the entire wall of the first part to provide the necessary strength. However, the present high cost of plastic makes this an uneconomical solution to this problem.
It is necessary that a lead extend from the plastic housing to permit completion of the test circuit. In some devices, instead of forming a separate opening in the housing through which the lead may extend, it is economical to provide the externally threaded section of the part of the probe with an axially extending elongated slot through which the lead may extend when the parts of the housing are assembled. The slot must be longer than the overlap between the parts, by a distance at least equal to the diameter of the lead, such that the lead can extend therethrough when the parts are assembled. Thus, the slot in the externally threaded section of the part through which the lead extends must be relatively long. When such a slot is employed, the wall of the externally threaded section is further weakened because the slot interrupts the inherent strength of the cylindrical wall.
The lead is internally connected to the base of the active incandescent bulb situated in the first part. In order to prevent strain on the connection between the lead and the base, a knot is formed in, or a washer-like member is affixed to, the section of the lead within the housing adjacent the opening from which the lead extends, thus preventing disconnection of the lead end if undue force is exerted on the lead, such as by pulling or the like.
As mentioned previously, circuit test devices of this type normally utilize an indicating device such as a small incandescent bulb, which is situated within the plastic housing and is energized when the circuit under test is operational. However, since the devices are designed to be portable for use in the field, it is desirable to incorporate a spare bulb therein, to be used in the event that the primary bulb ceases to function, and thus to provide facility for carrying the spare bulb within the housing itself. However, in order to do so, the housing must be provided with a means of retaining the spare bulb in a fixed position therein such that the bulb, which is relatively fragile, will not be broken or damaged by the handling of the device.
It is, therefore, a prime object of the present invention to provide a circuit test device comprising a plastic housing with walls of reduced thickness.
It is another object of the present invention to provide a circuit test device comprising a first part having an externally threaded section and a second part having an internally threaded section with a member insertable therebetween which serves to structurally reinforce the thin-walled, externally threaded section of the first part.
It is another object of the present invention to provide a circuit test device having means on the externally threaded section and on the member which cooperate, when the housing is assembled, to provide a radial channel through which a lead may extend and, at the same time, provide the lead with the necessary strain protection.
It is another object of the present invention to provide a circuit test device having facility to securely mount a spare bulb within the housing.
It is another object of the present invention to provide a circuit test device wherein a member is inserted between the housing parts, the member having a radially extending flange which is retained between the parts so as to properly position the member.
It is another object of the present invention to provide a circuit test device of the self-powered type wherein a battery is received within a cavity in the part having an internally threaded section, and wherein a member is inserted between the parts so as to increase the structural strength of the externally threaded section of the other part and permit electrical connection between the parts.
It is a further object of the present invention to provide a circuit test device which is formed of relatively simple, inexpensive parts which cooperate in a reliable and maintenance-free fashion.
In accordance with the present invention, a circuit test device is provided comprising a housing, a probe mounted to and extending from the housing, and a lead extending from the housing. The housing includes a first part having an externally threaded substantially cylindrical section. The externally threaded cylindrical section of the first part includes a wall having an internal diameter of a given dimension. A second part is provided having an internally threaded section adapted to cooperate with the externally threaded section of the first part to removably connect the parts. Inserted between the parts is a member having a substantially cylindrical portion with an external diameter substantially equal to the internal diameter of the wall of the externally threaded cylindrical section of the first part. This portion of the member is adapted to be received within the first part, adjacent the externally threaded cylindrical section thereof, when the housing is assembled, to enhance the structural strength of the externally threaded cylindrical section of the first part.
The wall of the externally threaded cylindrical section of the first part has a rim. The member further includes a flange extending radially from the edge of the substantially cylindrical portion thereof. The flange is adapted to be received adjacent the rim of the first part, when the housing is assembled, so as to properly locate the member with respect to the parts.
In a first preferred embodiment, the externally threaded cylindrical section of the first part is provided with a slot therein, the slot being elongated in a direction parallel to the axis of the housing. The member, and, more particularly, the substantially cylindrical portion thereof, is provided with a second slot therein. The slots are adapted to at least partially align, when the member is received within the first part, so as to form a substantially radial channel through the housing. A lead is adapted to extend through the channel. The lead comprises a conductive wire surrounded by an insulating coating or layer. The channel has an inner diameter which is smaller than the outer diameter of the insulating layer of the lead, but larger than the outer diameter of the wire. When the housing is assembled with the lead extending through the channel, the walls of the channel engage the insulating layer of the wire so as to provide strain protection therefor. Thus, the first part and the member each comprise means thereon which cooperate, when assembled, to engage the lead so as to prevent same from being displaced relative to the housing.
In the first preferred embodiment, the member additionally comprises means for mounting a spare bulb thereon. The bulb mounting means extends from the member in the direction of the second part when the housing is assembled. The second part preferably comprises a cavity therein. The bulb mounting means extending from the member is received within the cavity in the second part when the housing is assembled.
In a second preferred embodiment, a battery or similar power source is situated within the cavity in the second part of the housing, so as to make the test device self-powered. In this embodiment, the structure of the first part and, particularly, the externally threaded section thereof, is altered to eliminate the slot therein, as is the cylindrical portion of the member so as to eliminate the slot therein. In addition, the spare bulb mounting means, present on the member in the first embodiment, is eliminated so as to provide room for the battery and permit electrical connection of the battery, through the member and to the first part of the housing.